It is a well known fact that pupillary size changes with age. This is of some significance, for example, for amateur astronomers or late evening bird watchers in choosing a proper binocular lens [ ;-) ]. The scatterplot above shows the pupillary size in darkness of 1.263 subjects (the richest data pool on this subject so far). It can be easily concluded that the pupil has a tendency to become smaller with age, but the individual variation at a given age class is enormous.
There is a pupil research laboratory at the Tübingen Eye Hospital